1. Field of the Invention
The present invention relates to a bearing unit which is fixed to the circumferential surface of an output shaft at a vicinity of one end thereof and makes an eccentric motion by rotation of the output shaft, and an electric pump using this bearing unit.
2. Description of the Related Art
Conventionally, an electric pump, for example, an oil pump used in an automobile braking device, is driven by causing a rolling contact bearing (such as a ball bearing and a roller bearing), which supports an output shaft, to make an eccentric motion. There are various eccentric motion structures of a bearing for an output shaft used in such an electric pump. By way of example, the following types of structures are known: a structure in which an output shaft of a motor is formed by cutting work so that it may be brought into a state of eccentricity and a bearing whose inner and outer rings are concentric with each other is attached to the output shaft of the motor; a structure in which an inner ring of a bearing is decentered without causing an output shaft in itself to be decentered; a structure in which an eccentric bush is fitted into an inner ring through a hole in a bearing, and the like.
An example of other structures is disclosed in Japanese Utility Model Laid-Open No. 2-146220 (1990). According to this structure, an axis of an inner ring of a bearing which supports an output shaft is decentered from an axis of the output shaft, and this decentering causes a rotational motion of the output shaft to be converted to an eccentric motion.
A further example is disclosed in Japanese Utility Model Laid-Open No. 3-41176 (1991) . In this structure, inner and outer rings of a bearing are concentric with each other without causing the inner ring to be decentered from an axis of the outer ring. A decentered eccentric ring is fitted into a shaft center portion of the inner ring and the output shaft is fitted into a through hole of this eccentric ring, thereby causing a rotational motion of the output shaft to be converted to an eccentric motion.
However, in either case, unbalance arises from the axes of the output shaft and the bearing being decentered from each other and in a static state in which a motor is not being driven. This necessarily causes an unbalance in a dynamic state in which the motor is being driven. Therefore, these conventional structures have problems in that unbalance in a dynamic state causes vibration and the like, so that the bearing, the drive source of the motor, and the like may be damaged, and that the noise of an electric pump is increased.